Photolithography is widely used in semiconductor industry to fabricate electronic devices. Photolithography is a process which uses light to transfer a geometric pattern from a photomask to a substrate such as a silicon wafer. In a photolithography process, a photoresist layer is first formed on the substrate. The substrate is baked to remove any solvent remained in the photoresist layer. The photoresist is then exposed through a photomask with a desired pattern to a source of actinic radiation. The radiation exposure causes a chemical reaction in the exposed areas of the photoresist and creates a latent image corresponding to the mask pattern in the photoresist layer. The photoresist is next developed in a developer solution, usually an aqueous base solution, to form a pattern in the photoresist layer. The patterned photoresist can then be used as a mask for subsequent fabrication processes on the substrate, such as deposition, etching, or ion implantation processes.
Two types of photoresist have been used in photolithography: positive resist and negative resist. A positive resist is initially insoluble in the developer solution. After exposure, the exposed region of the resist becomes soluble in the developer solution and is then selectively removed by the developer solution during the subsequent development step. The unexposed region of the positive resist remains on the substrate to form a pattern in the photoresist layer. The selective removal of the exposed region of a photoresist is thus called “positive development”.
A negative resist behaves in the opposite manner. The negative resist is initially soluble in the developer solution. Exposure to radiation typically initiates a crosslinking reaction which causes the exposed region of the negative resist to become insoluble in the developer solution. During the subsequent development step, the unexposed region of the negative resist is selectively removed by the developer solution, leaving the exposed region on the substrate to form a pattern. Contrary to the “positive development”, a “negative development” refers to a process that selectively removes the unexposed region of a photoresist.
Most commercial photoresists for 193 nm photolithography are positive resists. However, as semiconductor ground rule continues to shrink, it has become ever more challenging to print small features, especially spaces such as trenches and vias of small dimensions using traditional positive resist with aqueous base developer due to the poor optical image contrast of the dark field masks used to create the trenches and vias. Therefore, there is a need for a photoresist composition and a pattern forming method that can print small features, particularly spaces of small dimensions.